The present invention relates to a technique for downsizing a water-gas shift reactor used in a mobile PEM fuel cell system.
Fuel cells electrochemically produce electricity from reactants supplied externally thereto, and have been proposed for many applications including mobile electric vehicle power plants to replace, or supplement, internal combustion engines. Hydrogen is often used as the fuel, and is supplied to the fuel cell""s anode. Oxygen (as air) is used as the oxidant and is supplied to the cell""s cathode. For mobile (e.g. vehicular) applications, the hydrogen fuel may be derived from liquid hydrocarbon fuels (e.g., methanol or gasoline) in a catalytic fuel processing reactor. For example, in the case of methanol, methanol and water (vapors) are reacted under isothermal conditions in a catalytic reactor known as a steam reformer that generates hydrogen and carbon dioxide according to the following ideal endothermic reaction:
CH3OH+H2Oxe2x86x92CO2+3H2 
This reaction is carried out within a steam reformer that is heated by exhaust gases from a methanol-fired and/or hydrogen-fired combuster, and yields a reformate gas comprising hydrogen, carbon dioxide, carbon monoxide, and water. One such reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh, and one such combuster is described in U.S. Pat. No. 6,232,005 issued May 15, 2001 and U.S. Pat. No. 6,077,620 issued Jun. 20, 2000 in the name of William Pettit, and assigned to General Motors Corporation, assignee of the present invention. Gasoline is a more complex reaction and may be reacted in a so-called autothermal reactor which comprises a partial oxidation (POX) reactor upstream of a steam reformer to partially oxidize the gasoline before steam reforming.
Unfortunately, the reformate/effluent exiting the steam reformer contains untoward amounts of carbon monoxide that is toxic to the catalyst in the fuel cell and must be removed, or at least reduced to very low concentrations (i.e., less than about 0.00005 mole fraction). It is known that the carbon monoxide, CO, content of the reformate can be reduced by the so-called xe2x80x9cwater-gas shiftxe2x80x9d reaction that can either take place within itself (depending on the operating conditions of the reformer), or, more typically, in one or more separate shift reactors located downstream from the reformer. In the water-gas shift reaction, water (i.e., steam) reacts with the carbon monoxide in the reformate according to the following ideal exothermic shift reaction:
CO+H2Oxe2x86x92CO2+H2 
In one known arrangement, a first shift reactor (i.e., in a two-reactor series) is a high-temperature, adiabatic shift reactor in which the reformate enters at a first temperature and exits at a somewhat higher temperature. Thereafter, the reformate is cooled and enters a second shift reactor which is an isothermal, low temperature shift reactor in which the inlet and outlet temperatures of the reformate is essentially the same. Shift reactor(s) comprise(s) a housing containing a catalyst bed through which the reformate flows, and to which steam is added. The first, or high temperature, shift reactors operate at about 350xc2x0 C.-450xc2x0 C., and typically use a non-noble metal catalyst such as a mixture of Fe3O4 and Cr2O3 (i.e., about 55 wt % Fe and 6% Cr). The second, or low temperature, shift reactors, on the other hand, operate at about 200xc2x0 C.-260xc2x0 C., and use a non-noble metal catalyst such as Cuxe2x80x94ZnOxe2x80x94Al2O3, or Cuxe2x80x94ZnOxe2x80x94Cr2O3. Some CO still survives the water-gas shift reaction.
CO concentration in the reformate must be reduced to below 0.00005 mole fraction before the reformate can be used in the fuel cell without poisoning the fuel cell catalyst. It is known to further reduce the CO content of H2-rich reformate exiting a shift reactor by selectively reacting it with air at a temperature of about 210xc2x0 C.-260xc2x0 C. in a so-called xe2x80x9cPrOxxe2x80x9d (i.e., preferential oxidation) reaction that is carried out in a PrOx reactor having a noble metal catalyst. In the PrOx reactor, the air preferentially oxidizes the CO, in the presence of the H2, but without consuming/oxidizing substantial quantities of the H2. The PrOx reaction is exothermic and proceeds as follows:
CO+{fraction (1/20)}2xe2x86x92CO2 
When the system reaches steady state, and the CO level is low enough, the PrOx reactor effluent is supplied to the fuel cell. Before the CO level is low enough, the PrOx effluent is shunted around the fuel cell for temporary use elsewhere in the system.
Vehicular fuel cell power plants need to be as compact as possible. Unfortunately, the water-gas-shift reactor is generally quite large because it requires a significant amount of catalyst. Much of the catalyst is needed toward the later half of the reactor (i.e., in the direction of reformate flow through the reactor) where the concentration of the CO is lowest and closer to equilibrium, and hence requires a significant amount of catalyst to effect the final stages of CO removal. This large amount of catalyst adds to the volume and cost of the shift reactor and adds to the time required to bring the reactor up to its preferred superambient operating temperature during start-up of the fuel cell system.
The present invention contemplates a technique for reducing the size of, and amount of catalyst needed for, a water-gas-shift reactor without compromising the ability of the reactor to remove a sufficient amount of the CO that the effluent can be treated in a PrOx reactor to render it non-toxic to a fuel cell. More specifically, the present invention contemplates process and apparatus for injecting a small amount of oxygen into the tail section of a water-gas-shift reactor operating under steady state conditions to consume the low levels of CO in the reformate therein and thereby eliminate excess catalyst otherwise needed to effect the water-gas shift reactor thereat. By tail section is meant that portion of the water-gas-shift-reactor that (1) is downstream of the reactor""s inlet, and (2) receives reformate from an upstream portion of the reactor that has a CO content of about 2% by volume
More specifically yet, the present invention relates to a fuel cell system comprising a PEM fuel cell for electrochemically reacting a hydrogen-rich fuel gas stream with oxygen (i.e., from air) to produce electricity. The hydrogen-rich fuel/gas stream supplied to the fuel cell has a sufficiently low concentration of carbon monoxide as to be tolerable by the fuel cell (i.e., less than about 0.00005 mole fraction, or 50 ppm). The fuel gas stream is produced from a liquid hydrocarbon (e.g., methanol or gasoline) in a first catalytic reactor (e.g. a steam reformer) located upstream of the fuel cell. The output from the first reactor has a concentration of carbon monoxide that is too high to be used in the fuel cell. A low temperature water-gas-shift reactor is therefore positioned intermediate the first catalytic reactor and the fuel cell and serves to reduce higher carbon monoxide concentrations exiting the first catalytic reactor to a lower level closer to that tolerable by the fuel cell. The low temperature water-gas-shift reactor will preferably be an isothermal reactor having an internal heat exchanger construction suitable to removing reactor-generated heat therefrom. The low temperature water-gas-shift reactor may, or may not, be preceded by a high temperature shift reactor. Each shift reactor comprises a housing encasing a catalyst bed that promotes the reaction between steam and the carbon monoxide in the fuel gas exiting the first reactor at a superambient operating temperature established for the reactor to most efficiently promote the reaction when the shift reactor is operating under normal operating conditions.
The low temperature shift reactor comprises an upstream catalyst portion adjacent the reactor inlet end (i.e. where reformate enters the reactor), and a downstream catalyst portion, or tail section, adjacent the reactor outlet end. The tail section is that portion of the reactor that begins at the point in the catalyst bed in the direction of flow therethrough where the concentration of the CO in the reformate is about 2% by volume. In a typical low temperature water-gas-shift reactor this occurs about halfway through the bed.
In accordance with the present invention, the tail section is equipped with an oxygen distributor that distributes O2 (as air) throughout the catalyst bed in the tail section for consuming the carbon monoxide in the tail section. The oxygen injected into the tail section exothermically reacts with the carbon monoxide (and hydrogen as well) in the reformate passing through the tail section of the catalyst bed to significantly drop its concentration without the need for the amount of catalyst otherwise needed for steam to effect the same low CO level.
The overall system includes: (1) a hydrogen-fueled PEM fuel cell; (2) a catalytic reactor upstream of the fuel cell that, under normal operating conditions, produces a hydrogen-containing fuel gas stream from a liquid hydrocarbon for fueling the fuel cell, which stream is contaminated with a first concentration of carbon monoxide that is too high for the fuel cell to tolerate; and (3) at least one shift reactor intermediate the catalytic reactor and the fuel cell for reducing the concentration of the carbon monoxide in the fuel gas stream to a second concentration less than the first concentration, and closer to that tolerable by the fuel cell, when the system is operating under normal operating conditions. The shift reactor(s) comprise(s) a housing encasing a catalyst bed for reacting the carbon monoxide in the fuel gas stream with water (i.e. steam) at a superambient operating temperature established for the most effective use of the catalyst when the system is operating under normal operating conditions. The reactor has an upstream portion adjacent the reactor""s inlet and a downstream portion (aka tail section) adjacent the reactor""s outlet. The tail section is that portion of the reactor that is downstream of the upstream portion and begins where the reformate has a CO concentration of about 2% by volume. In accordance with the process aspect of the invention, O2 (i.e. as air) is injected into the tail section of the reactor to consume the CO therein and reduce the CO concentration in the water-gas-shift reactor effluent to levels that can readily be removed to non-toxic levels in a PrOx reactor. The O2+COxe2x86x92reaction occurs quickly on a significantly less volume of catalyst than would otherwise be required to accomplish the same result using only the water-gas-shift reaction alone.